Honestly they'll get their just desserts scootering on the sidewalk. Those small wheels will jam on the smallest imperfections and rocks throwing the rider face first over the handlebars.

I see a lot of people mentioning razor blades, and I just want to mention that you can get plastic razor blades from hardware stores, like Tags, if you're afraid of scratching the glass. After you get the bulk of it off with the razor blade, use isopropyl alcohol to attack the remaining adhesive.